The present invention relates generally to refrigeration system including a variable speed fan that changes the speed of airflow through the evaporator as environmental conditions change.
Chlorine containing refrigerants have been phased out in most of the world due to their ozone destroying potential. Hydrofluoro carbons (HFCs) have been used as replacement refrigerants, but these refrigerants still have high global warming potential. “Natural” refrigerants, such as carbon dioxide and propane, have been proposed as replacement fluids. Carbon dioxide has a low critical point, which causes most refrigeration systems utilizing carbon dioxide as a refrigerant to run partially above the critical point, or to run transcritical, under most conditions. The pressure of any subcritical fluid is a function of temperature under saturated conditions (when both liquid and vapor are present). However, when the temperature of the fluid is higher than the critical temperature (supercritical), the pressure becomes a function of the density of the fluid.
In a refrigeration system, the refrigerant is compressed to a high pressure in the compressor. In a gas cooler heat is removed from the high pressure refrigerant and is transferred to a fluid medium, such as water. The refrigerant is expanded to a low pressure in an expansion device. The refrigerant then passes through an evaporator and accepts heat from ambient air. The low pressure refrigerant then re-enters the compressor completing the cycle.
A refrigeration system can have a wide range of operating conditions. For example, the temperature of the ambient air at the evaporator inlet can vary from approximately −10° F. in the winter to approximately 120° F. in the summer. Therefore, the refrigerant evaporating temperature can vary from approximately −20° F. to approximately 100° F. Therefore, the mass flow rate of the refrigerant and the heating capacity of the system in the summer can be eight to ten times as high as the heating capacity in the winter, and the heating capacity of the system in the summer can be four to five times as high as the heating capacity in the winter. The gas cooler and the evaporator are able to handle the maximum and minimum refrigerant flow and heating capacity. However, they are always optimized when operating under the seasonal average condition, such as when the ambient air temperature is at 50° F.